tcpdump

Dump traffic on a network

Syntax
tcpdump [ -AdDefgIKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ] [ -i interface ]
[ -m module ] [ -M secret ] [ -r file ] [ -s snaplen ]
[ -T type ] [ -w file ] [ -W filecount ] [ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z user ] [ expression ]
Options:
-A Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
-B Set the operating system capture buffer size to buffer_size.
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file is currently larger than
file_size and, if so, close the current savefile and open a new one. Savefiles after the
first savefile will have the name specified with the -w flag, with a number after it, starting
at 1 and continuing upward. The units of file_size are millions of bytes (1,000,000 bytes,
not 1,048,576 bytes).
-d Dump the compiled packet-matching code in a human readable form to standard output and stop.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (preceded with a count).
-D Print the list of the network interfaces available on the system and on which tcpdump can capture
packets. For each network interface, a number and an interface name, possibly followed
by a text description of the interface, is printed. The interface name or the number can be
supplied to the -i flag to specify an interface on which to capture.
This can be useful on systems that don't have a command to list them (e.g., Windows systems,
or UNIX systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems,
where the interface name is a somewhat complex string.
The -D flag will not be supported if tcpdump was built with an older version of libpcap that
lacks the pcap_findalldevs() function.
-e Print the link-level header on each dump line.
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and
contain Security Parameter Index value spi. This combination may be repeated with comma or
newline seperation.
Note that setting the secret for IPv4 ESP packets is supported at this time.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default
is des-cbc. The ability to decrypt packets is only present if tcpdump was compiled with cryptography
enabled.
secret is the ASCII text for ESP secret key. If preceeded by 0x, then a hex value will be
read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The option is only for debugging purposes,
and the use of this option with a true `secret' key is discouraged. By presenting IPsec
secret key onto command line you make it visible to others, via ps(1) and other occasions.
In addition to the above syntax, the syntax file name may be used to have tcpdump read the
provided file in. The file is opened upon receiving the first ESP packet, so any special permissions
that tcpdump may have been given should already have been given up.
-f Print `foreign' IPv4 addresses numerically rather than symbolically (this option is intended
to get around serious brain damage in Sun's NIS server -- usually it hangs forever translating
non-local internet numbers).
The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the interface
on which capture is being done. If that address or netmask are not available, available,
either because the interface on which capture is being done has no address or netmask or
because the capture is being done on the Linux "any" interface, which can capture on more than
one interface, this option will not work correctly.
-F Use file as input for the filter expression. An additional expression given on the command
line is ignored.
-g Do not insert line break after IP header in verbose mode for easier parsing.
-G If specified, rotates the dump file specified with the -w option every rotate_seconds seconds.
Savefiles will have the name specified by -w which should include a time format as defined by
strftime(3). If no time format is specified, each new file will overwrite the previous.
If used in conjunction with the -C option, filenames will take the form of `filecount'.
-i Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest
numbered, configured up interface (excluding loopback). Ties are broken by choosing the
earliest match.
On Linux systems with 2.2 or later kernels, an interface argument of ``any' can be used to
capture packets from all interfaces. Note that captures on the ``any'' device will not be
done in promiscuous mode.
If the -D flag is supported, an interface number as printed by that flag can be used as the
interface argument.
-I Put the interface in "monitor mode"; this is supported only on IEEE 802.11 Wi-Fi interfaces,
and supported only on some operating systems.
Note that in monitor mode the adapter might disassociate from the network with which it's
associated, so that you will not be able to use any wireless networks with that adapter. This
could prevent accessing files on a network server, or resolving host names or network
addresses, if you are capturing in monitor mode and are not connected to another network with
another adapter.
-K Don't attempt to verify TCP, UDP and IP checksums. This is useful for interfaces that perform
the checksum calculation in hardware; otherwise, all outgoing checksums will be flagged as
bad.
-l Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,
`tcpdump -l | tee dat' or `tcpdump -l > dat & tail -f dat'.
-L List the known data link types for the interface and exit.
-m Load SMI MIB module definitions from file module. This option can be used several times to
load several MIB modules into tcpdump.
-M Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5
MD5 option (RFC 2385), if present.
-n Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
-N Don't print domain name qualification of host names. E.g., if you give this flag then tcpdump
will print `nic' instead of `nic.ddn.mil'.
-O Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in
the optimizer.
-p Don't put the interface into promiscuous mode. Note that the interface might be in promiscuous
mode for some other reason; hence, `-p' cannot be used as an abbreviation for `ether host
{local-hw-addr} or ether broadcast'.
-q Quick (quiet?) output. Print less protocol information so output lines are shorter.
-R Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified,
tcpdump will not print replay prevention field. Since there is no protocol version field in
ESP/AH specification, tcpdump cannot deduce the version of ESP/AH protocol.
-r Read packets from file (which was created with the -w option). Standard input is used if file
is `-'.
-S Print absolute, rather than relative, TCP sequence numbers.
-s Snarf snaplen bytes of data from each packet rather than the default of 64K bytes. Packets
truncated because of a limited snapshot are indicated in the output with `[|proto]', where
proto is the name of the protocol level at which the truncation has occurred. Note that taking
larger snapshots both increases the amount of time it takes to process packets and,effectively,
decreases the amount of packet buffering. This may cause packets to be lost. You
should limit snaplen to the smallest number that will capture the protocol information you're
interested in. Setting snaplen to 0 means use the required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted the specified type. Currently known
types are aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc
(Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications
control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Protocol),
, vat (Visual Audio Tool), and wb (distributed White Board).
-t Don't print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-ttt Print a delta (micro-second resolution) between current and previous line on each dump line.
-tttt Print a timestamp in default format proceeded by date on each dump line.
-ttttt Print a delta (micro-second resolution) between current and first line on each dump line.
-u Print undecoded NFS handles.
-U Make output saved via the -w option `packet-buffered'; i.e., as each packet is saved, it
will be written to the output file, rather than being written only when the output buffer
fills.
The -U flag will not be supported if tcpdump was built with an older version of libpcap that
lacks the pcap_dump_flush() function.
-v When parsing and printing, produce (slightly more) verbose output. For example, the time to
live, identification, total length and options in an IP packet are printed. Also enables
additional packet integrity checks such as verifying the IP and ICMP header checksum.
When writing to a file with the -w option, report, every 10 seconds, the number of packets
captured.
-vv Even more verbose output. For example, additional fields are printed from NFS reply packets,
and SMB packets are fully decoded.
-vvv Even more verbose output. For example, telnet SB ... SE options are printed in full. With -X
Telnet options are printed in hex as well.
-w Write the raw packets to file rather than parsing and printing them out. They can later be
printed with the -r option. Standard output is used if file is `-'.
-W Used in conjunction with the -C option, this will limit the number of files created to the
specified number, and begin overwriting files from the beginning, thus creating a 'rotating'
buffer. In addition, it will name the files with enough leading 0s to support the maximum
number of files, allowing them to sort correctly.
Used in conjunction with the -G option, this will limit the number of rotated dump files that
get created, exiting with status 0 when reaching the limit. If used with -C as well, the
behavior will result in cyclical files per timeslice.
-x When parsing and printing, in addition to printing the headers of each packet, print the data
of each packet (minus its link level header) in hex. The smaller of the entire packet or
snaplen bytes will be printed. Note that this is the entire link-layer packet, so for link
layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer
packet is shorter than the required padding.
-xx When parsing and printing, in addition to printing the headers of each packet, print the data
of each packet, including its link level header, in hex.
-X When parsing and printing, in addition to printing the headers of each packet, print the data
of each packet (minus its link level header) in hex and ASCII. This is very handy for
analysing new protocols.
-XX When parsing and printing, in addition to printing the headers of each packet, print the data
of each packet, including its link level header, in hex and ASCII.
-y Set the data link type to use while capturing packets to datalinktype.
-z Used in conjunction with the -C or -G options, this will make tcpdump run " command file "
where file is the savefile being closed after each rotation. For example, specifying -z gzip
or -z bzip2 will compress each savefile using gzip or bzip2.
Note that tcpdump will run the command in parallel to the capture, using the lowest priority
so that this doesn't disturb the capture process.
And in case you would like to use a command that itself takes flags or different arguments,
you can always write a shell script that will take the savefile name as the only argument,
make the flags & arguments arrangements and execute the command that you want.
-Z Drops privileges (if root) and changes user ID to user and the group ID to the primary group
of user.
This behavior can also be enabled by default at compile time.
expression
selects which packets will be dumped. If no expression is given, all packets on the net will
be dumped. Otherwise, only packets for which expression is `true' will be dumped.
For the expression syntax, see pcap-filter(4).
Expression arguments can be passed to tcpdump as either a single argument or as multiple arguments,
whichever is more convenient. Generally, if the expression contains Shell metacharacters,
it is easier to pass it as a single, quoted argument. Multiple arguments are concatenated
with spaces before being parsed.

Tcpdump prints out a description of the contents of packets on a network interface that match the
boolean expression. It can also be run with the -w flag, which causes it to save the packet data to
a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file
rather than to read packets from a network interface. In all cases, only packets that match expression will be processed by tcpdump.

Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a
SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or a
SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture
packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets
have been processed.

Typing ctrl-q and ctrl-s will pause and unpause the output.

When tcpdump finishes capturing packets, it will report counts of:

packets `captured' (this is the number of packets that tcpdump has received and processed);

packets `received by filter' (the meaning of this depends on the OS on which you're running
tcpdump, and possibly on the way the OS was configured - if a filter was specified on the com-mand command line, on some OSes it counts packets regardless of whether they were matched by the filter expression and, even if they were matched by the filter expression, regardless of whether
tcpdump has read and processed them yet, on other OSes it counts only packets that were
matched by the filter expression regardless of whether tcpdump has read and processed them
yet, and on other OSes it counts only packets that were matched by the filter expression and
were processed by tcpdump);

packets ``dropped by kernel'' (this is the number of packets that were dropped, due to a lack
of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the
OS reports that information to applications; if not, it will be reported as 0).

On platforms that support the SIGINFO signal, such as most BSDs (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts when it receives a SIGINFO signal (generated, for example, by typing your `status' character, typically control-T, although on some platforms, such as
Mac OS X, the `status' character is not set by default, so you must set it with stty(1) in order to
use it) and will continue capturing packets.

Reading packets from a network interface may require that you have special privileges; see the pcap
(3PCAP) man page for details. Reading a saved packet file doesn't require special privileges.

Example:

To print all tcp traffic for interface "en1" (typically the Airport):
sudo tcpdump -i en1
To print all packets arriving at or departing from sundown:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup: (note that the expression is quoted to pre-vent prevent
vent the shell from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net,
this stuff should never make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a
non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not,
for example, SYN and FIN packets and ACK-only packets. (IPv6 is left as an exercise for the reader.)
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

“Facts which at first seem improbable will, even on scant explanation, drop the cloak which has hidden them and stand forth in naked and simple beauty” ~ Galileo Galilei